The processing of a microwave, for example received by a satellite, requires the development of specific components allowing the propagation, the amplification and the filtering of this wave.
For example, a microwave received by a satellite must be amplified before being returned to the ground. This amplification is possible only by separating all the frequencies received into channels, each corresponding to a given frequency band. The amplification is then carried out channel by channel. The separation of the channels requires the development of band-pass filters.
The development of satellites and the increased complexity of the signal processing to be carried out, for example a reconfiguration of the channels in flight, has led to the need to use frequency-tunable band-pass filters, that is to say filters for which it is possible to adjust the central filtering frequency widely named the tuning frequency of the filter.
One of the known technologies of tunable band-pass filters in the microwave domain is the use of passive semiconductor components, such as PIN diodes, continuously variable capacitors or capacitive switches. Another technology is the use of MEMS (for microelectromechanical systems) of the ohmic or capacitive type.
These technologies are complex, they consume electrical power and are not very reliable. These solutions are also limited to the level of signal power processed. In addition, frequency tunability results in a significant deterioration in the performance of the filter, such as its quality factor Q.
Furthermore, the technology of filters based on dielectric elements is known. It makes it possible to produce non-tunable band-pass filters.
FIG. 1 describes an example of a filter based on dielectric elements for non-tunable microwaves.
An input excitation means 10 inserts the wave into the cavity; this element is typically a conductive medium such as a coaxial cable (or probe).
The cavity 13 is a closed cavity consisting of metal, typically aluminum or a metal alloy such as Invar.
An output excitation means 11, typically a conductive medium such as a coaxial cable (or probe) makes it possible to take the wave out of the cavity.
The dielectric element 12 is round or square in shape and placed inside the metal cavity 13. The dielectric material is typically zirconia, alumina or barium magnesium tantalate (“BMT”).
A filter typically comprises at least one resonator comprising a metal cavity and a dielectric element. A resonance mode of the filter corresponds to a particular distribution of the electromagnetic field which is excited at a particular frequency.
A band-pass filter allows the propagation of a wave over a certain frequency range and attenuates this wave for the other frequencies. This therefore defines a bandwidth and a central frequency of the filter. For frequencies around its central frequency, a band-pass filter has a high transmission and a weak reflection.
In order to increase their selectivity, that is to say their capacity to attenuate the signal outside the bandwidth, these filters may be composed of a plurality of resonators that are coupled together.
The central frequency and the bandwidth of the filter depend both on the geometry of the cavities and of the dielectric elements, and on the coupling together of the resonators as well as the couplings to the input and output excitation means of the filter.
Coupling means are for example apertures or slots which may otherwise be known as irises, electrical or magnetic probes or microwave lines.
The bandwidth of the filter is characterized in different ways depending on the nature of the filter.
The parameter S is a parameter which reports the performance of the filter in terms of reflection and transmission, respectively. S11 or S22 corresponds to a measurement of the reflection and S12 or S21 to a measurement of the transmission.
A filter performs a filtering function. This function may usually be approximated via mathematical models (iterative functions such as Chebychev, Bessel, etc. functions). These functions are usually founded on polynomial ratios.
For a filter performing a filtering function of the Chebychev or generalized Chebychev type, the bandwidth of the filter is determined at equal ripple of the S11 (or S22), for example at 15 dB or 20 dB of reduction of the reflection relative to its out-band level. For a filter performing a function of the Bessel type, the frequency band corresponding to a bandwidth of −3 dB (when S21 crosses S11) is determined to be the pass band.
An example of a characteristic of the parameters S11 and S12 of a filter is illustrated in FIG. 2. The curve 21 corresponds to the reflection S11 in dB of the wave on the filter as a function of its frequency in GHz. The equal-ripple bandwidth at 20 dB of reflection is designated by the numeral 26. The filter has a central frequency fc corresponding to the frequency of the middle of the bandwidth. The curve 22 of FIG. 2 corresponds to the transmission S12 in dB of the filter as a function of the frequency in GHz. The filter therefore allows to pass a signal of which the frequency is situated in the bandwidth, but the signal is nevertheless attenuated by the losses of the filter.
The tuning of the filter making it possible to obtain a maximum of transmission for a given frequency band may be difficult to achieve and depends on all of the parameters of the filter. It is also dependent on the temperature.
In order to adjust the filter to obtain a precise central frequency of the filter, the resonance frequencies of the resonators of the filter may be very slightly modified with the aid of metal screws, but this method, carried out empirically, is very costly in time and provides only a very slight frequency tunability, typically of the order of a few %. In this case, the objective is not tunability but the obtaining of a precise value of the central frequency, and it is desired to obtain a reduced frequency sensitivity of each resonator with respect to the depth of the screw.
The circular or square symmetry of the resonators simplifies the design of the filter and the selection of the mode (TE for Transverse Electric or TM for Transverse Magnetic) that is propagated in the filter.
U.S. Pat. No. 7,705,694 describes a bandwidth-tunable filter consisting of a plurality of dielectric resonators coupled together, of non-uniform shape radially and uniform shape on an axis z perpendicular to the direction of propagation. Each resonator is capable of carrying out a rotation around the axis z between two positions, which induces a change of value of the width of the bandwidth, typically from 51 Mz to 68 Mz. This device allows tunability on the value of the width of the bandwidth of the filter, but not on its central frequency.